The semiconductor industry has seen tremendous advances in technology in recent years that have permitted dramatic increases in circuit density and complexity, and equally dramatic decreases in power consumption and package sizes. Present semiconductor technology now permits single-chip microprocessors with many millions of transistors, operating at speeds of tens (or even hundreds) of MIPS (millions of instructions per second), to be packaged in relatively small, air-cooled semiconductor device packages. A by-product of such high density and high functionality in semiconductor devices has been the demand for increased numbers of external electrical connections to be present on the exterior of the die and on the exterior of the semiconductor packages that receive the die, for connecting the packaged device to external systems, such as a printed circuit board.
In the past, the die and package were first attached and then were wire bonded. Wire bonding has many problems. The problems include limiting the number of pads and placement of the pads on the die, and a chance of electrical performance problems or shorting if the wires come too close to each other. As a result, wire bonding has given way to ball grid array packages in many applications.
Ball grid arrays (“BGAs”) are an array of solder bumps or balls that cover the surface of the die or semiconductor package and are used to connect the die and the semiconductor package. A typical BGA package is characterized by a large number of solder balls disposed in an array on a surface of the package. It is not uncommon to have hundreds of solder balls in an array. The BGA package is assembled to a matching array of conductive pads. The pads are connected to other devices within a substrate or circuitry on a circuit board. Heat is applied to reflow the solder balls (bumps) on the package, thereby wetting the pads on the substrates and, once cooled, forming electrical connections between the package and the semiconductor device contained in the package and the substrate.
BGAs have the advantage of providing more connections between the die and the semiconductor package. The BGAs also have the advantage that the size of the balls or bumps can be made smaller to provide a higher density of solder bumps or balls or a greater number of connections from a die. BGAs are formed by placing an amount of solder on a solder pad and heating the solder to a melting point. The surface tension associated with the liquid solder causes the solder to form a solder ball. The solder ball retains its shape as it cools to form a solid solder ball or bump.
Two basic types of BGA pads for interconnection include a metal defined flat pad (MD) type BGA, and a solder mask defined flat pad (SMD) type BGA. Both of these two types of BGA pads are shown in FIGS. 1 and 2 of the attached drawings. The solder adheres to the metal of the pad and not to the non-metallic substrate. In the metal defined flat pad type BGA, shown in FIG. 1, a metal pad 100 is initially formed on a substrate 110. Molten solder is then applied to the metal pad 100. The surface tension associated with the liquid or molten solder 120 forms the solder into a solder ball 120. The solder 120 adheres to the entire metal pad 100. In the solder mask flat pad type of BGA, shown in FIG. 2, a metal pad 100 is initially formed on a substrate 110. Then a solder mask 200 is laid down to further limit the opening portion of the metal pad 100 that the solder adheres to. In other words, the solder of the solder ball 120 formed adheres only to the uncovered or unmasked portion 112 of the metal pad. The solder adheres to the metal and not the solder mask. The surface tension associated with the liquid or molten solder 120 forms the solder into a solder ball 120. The solder 120 adheres to the uncovered portion 112 of the metal pad 100.
In the metal defined flat pad type of BGA, there may be a weakness referred to as pad cratering. Pad cratering occurs when the solder ball undergoes mechanical bending stress. Adding a solder mask lessens pad cratering. FIG. 3 is a cross-sectional view of a component attached to a printed circuit board using solder mask defined BGA. As shown by arrows referenced as 320 and 322, as the solder mask is formed by etching, a point is formed in the solder mask 200. In the solder mask defined flat pad type BGA, there may be a weakness due to stress concentration that becomes problematic as the solder ball undergoes thermal cycling. The points 320, 322 are formed along or near the top surface of the solder mask 200 and at the periphery of the BGA. The points 320, 322 are formed because the etchant used to remove solder mask material undercuts the remaining mask material. The points may then act to concentrate stress. After a certain amount of thermal cycling, the solder ball may fatigue or crack at the point of stress. In addition, the solder mask defined flat type pad also has relatively high variation in pad diameter and size because of difficulties and limitations associated with the solder masking process. When the solder balls are of different diameters unintended opens may occur since the tops of the solder balls are not substantially coplanar. The solder balls may be misaligned after placement on a printed circuit board or substrate prior to solder reflow.
The description set out herein illustrates the various embodiments of the invention and such description is not intended to be construed as limiting in any manner.